Introduction
High power laser materials processing in the infra-red region invariably involves use of a focused laser beam which is produced either by refractive optical techniques, e.g. simple lenses, or by reflective optical elements, e.g. paraboloidal mirrors. These elements generate high intensity circularly symmetric energy distributions which can be used for a wide variety of materials processing applications including cutting, welding, marking and surface heat treatment. Conventionally, processing of materials is achieved either by translation of the focused spot using beam steering mirrors, or by the movement of the workpiece itself.
Laser mode
The transverse power intensity distribution of a laser beam depends on the 'mode' of the laser, and at best is Gaussian in form. Higher power lasers with more complex 'modes' produce beams with more complex energy distributions. In many applications the use of a single focused spot with cross-sectional intensity variations is not necessarily the most efficient or appropriate energy distribution for a particular laser process. For example, in laser hardening, a de-focused Gaussian spot is sometimes used. This distribution leads to a lenticular hardened profile where, in most cases, a large, uniformly hardened area is required. In such applications the use of specially tailored diffractive optical elements can make laser materials processing a more attractive alternative to conventional processing techniques.
Diffractive optics - background
Diffractive optical elements can be used to generate complex two- and three-dimensional energy distributions without the need for any moving components. Diffractive optics are widely utilised in low power applications, generally in the visible region, for applications such as optical interconnects, multiple focusing and image processing. However, their effective use in high power laser applications has been mainly limited to beam/fibre coupling, beam splitting, beam sampling, and Gaussian to flat-top conversion.
Uniquely tailored optics
TWI, in conjunction with Loughborough University (UK), has used a high power CO
2 laser to demonstrate the versatility of diffractive optics in producing energy distributions which are uniquely tailored to specific processes, materials and component geometries.
Diffractive optics use computer generated holographic patterns which, when illuminated with coherent radiation, diffract that energy into a mathematically specified distribution. The type of computer generated hologram used in the TWI/Loughborough work is the kinoform which transforms the phase (a property of laser beams not usually used in laser materials processing applications) of the illuminating beam and has the unique ability to diffract all illuminating radiation into a single diffraction image or 'order'. Therefore, kinoforms ideally have a diffraction efficiency of 100%.
Other types of computer generated hologram reconstruct not only the desired image but also additional unwanted orders. Its high efficiency makes the kinoform ideal for materials processing, where equipment costs are high and laser power wasted in an un-utilised image is not acceptable. The diffractive elements operate reflectively, phase changes are imposed on the illuminating beam by spatial variations in depth across a pattern produced on a suitable substrate.
Low cost technique
Commonly used techniques to fabricate highly efficient diffractive optical elements include step and repeat binary mask fabrication and direct-write electron beam lithography. The diffractive optics used at TWI have been fabricated using a low cost photo-lithographic technique. This method involves a single photo-lithographic exposure using a greyscale mask, i.e. a mask theoretically capable of an infinite number of 'diffracting levels'. The mask is produced by a commercial slide printer on photographic emulsion.
The diffractive optics have been tested using a range of fast axial flow and sealed tube CO2 lasers, according to the processing being undertaken. Optics have been designed for three specific industrial applications:
- simultaneous cutting and welding of plastic sheet
- laser transformation hardening of steel
- soldering of electronic components on a circuit board
Diffractive optics - applications
Simultaneous cutting and welding of plastic sheet
The benefits of welding and cutting plastics using lasers largely stem from the processing speeds available and the nature of the process. For high speed cutting, the laser beam is used at or near focus; for welding applications it is common to work in a defocused condition to produce a wide weld. By correct positioning of the focused spot, simultaneous cut-seal joints can be produced ( Fig.1.), but these are generally not particularly strong due to the narrow weld zone.
A third diffractive optic has been designed and manufactured to produce, simultaneously, a wide, low intensity energy distribution to make a weld but with a sharp, centre peak of intensity to produce a cut. When this optic was used to cut and weld two sheets of 0.1mm thick polypropylene, a consistent weld around 2.5mm wide was produced on each side of the cut (Fig.2.).
Laser transformation hardening
Lasers are not efficient as bulk heating devices, however, they are efficient at heating discrete areas very rapidly and are, therefore, suitable for certain transformation hardening applications. Use of a de-focused laser beam for transformation hardening, with its Gaussian distribution of energy, produces a very lenticular shaped transformed area, the case depth being largest in the centre where the applied power density is usually the highest that will prevent surface melting.
A diffractive optic has been manufactured to produce a line of uniform energy with peaks of higher energy superimposed at each end of line. Using this distribution, hardened tracks ~8mm wide have been produced with a very uniform case depth. The uniformity has been produced by a correct balance between the energy in the peaks at the extremes of the track and that in the centre. Specific hardness profiles on components of complex geometry have also been demonstrated.
Soldering of electronic components
Repetitious laser welding or soldering of electronic components (with complex leg arrangements) to circuit boards is a large, potential application for diffractive optics.
The advantage of a diffractive optic is that the laser beam can be split into a pattern of discrete spots to perform this operation without movement of either component or laser beam. A sixteen-point printed circuit board has been used as a test component to demonstrate discrete point laser soldering. A diffractive optic - designed to split the laser beam into 16, even intensity, but asymmetrically distributed, spots - was produced and successfully used to solder the leads to the board after manual application of solder paste to each pin. The potential for soldering a whole component in a single laser pulse has obvious advantages for mass production.
Conclusion
In the recent work at TWI, diffractive optical elements have been successfully used to produce tailored energy distributions which have been applied to industrial applications in the areas of surface engineering, plastics processing and printed circuit board soldering. In all cases, the processes concerned would have been impossible or extremely difficult using conventional reflective or refractive CO2 laser beam focusing systems.
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